﻿// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// TransformManyBlock.cs
//
//
// A propagator block that runs a function on each input to produce zero or more outputs.
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Diagnostics.Contracts;
using System.Linq;
using System.Threading.Tasks.Dataflow.Internal;
using System.Collections.ObjectModel;

namespace System.Threading.Tasks.Dataflow
{
	/// <summary>Provides a dataflow block that invokes a provided <see cref="System.Func{T,TResult}"/> delegate for every data element received.</summary>
	/// <typeparam name="TInput">Specifies the type of data received and operated on by this <see cref="TransformManyBlock{TInput,TOutput}"/>.</typeparam>
	/// <typeparam name="TOutput">Specifies the type of data output by this <see cref="TransformManyBlock{TInput,TOutput}"/>.</typeparam>
	[DebuggerDisplay("{DebuggerDisplayContent,nq}")]
	[DebuggerTypeProxy(typeof(TransformManyBlock<,>.DebugView))]
	public sealed class TransformManyBlock<TInput, TOutput> : IPropagatorBlock<TInput, TOutput>, IReceivableSourceBlock<TOutput>, IDebuggerDisplay
	{
		/// <summary>The target side.</summary>
		private readonly TargetCore<TInput> _target;
		/// <summary>
		/// Buffer used to reorder output sets that may have completed out-of-order between the target half and the source half.
		/// This specialized reordering buffer supports streaming out enumerables if the message is the next in line.
		/// </summary>
		private readonly ReorderingBuffer<IEnumerable<TOutput>> _reorderingBuffer;
		/// <summary>The source side.</summary>
		private readonly SourceCore<TOutput> _source;

		/// <summary>Initializes the <see cref="TransformManyBlock{TInput,TOutput}"/> with the specified function.</summary>
		/// <param name="transform">
		/// The function to invoke with each data element received.  All of the data from the returned <see cref="System.Collections.Generic.IEnumerable{TOutput}"/>
		/// will be made available as output from this <see cref="TransformManyBlock{TInput,TOutput}"/>.
		/// </param>
		/// <exception cref="System.ArgumentNullException">The <paramref name="transform"/> is null (Nothing in Visual Basic).</exception>
		[SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public TransformManyBlock(Func<TInput, IEnumerable<TOutput>> transform) :
			this(transform, null, ExecutionDataflowBlockOptions.Default)
		{ }

		/// <summary>Initializes the <see cref="TransformManyBlock{TInput,TOutput}"/> with the specified function and <see cref="ExecutionDataflowBlockOptions"/>.</summary>
		/// <param name="transform">
		/// The function to invoke with each data element received.  All of the data from the returned in the <see cref="System.Collections.Generic.IEnumerable{TOutput}"/>
		/// will be made available as output from this <see cref="TransformManyBlock{TInput,TOutput}"/>.
		/// </param>
		/// <param name="dataflowBlockOptions">The options with which to configure this <see cref="TransformManyBlock{TInput,TOutput}"/>.</param>
		/// <exception cref="System.ArgumentNullException">The <paramref name="transform"/> is null (Nothing in Visual Basic).</exception>
		/// <exception cref="System.ArgumentNullException">The <paramref name="dataflowBlockOptions"/> is null (Nothing in Visual Basic).</exception>
		[SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public TransformManyBlock(Func<TInput, IEnumerable<TOutput>> transform, ExecutionDataflowBlockOptions dataflowBlockOptions) :
			this(transform, null, dataflowBlockOptions)
		{ }

		/// <summary>Initializes the <see cref="TransformManyBlock{TInput,TOutput}"/> with the specified function.</summary>
		/// <param name="transform">
		/// The function to invoke with each data element received. All of the data asynchronously returned in the <see cref="System.Collections.Generic.IEnumerable{TOutput}"/>
		/// will be made available as output from this <see cref="TransformManyBlock{TInput,TOutput}"/>.
		/// </param>
		/// <exception cref="System.ArgumentNullException">The <paramref name="transform"/> is null (Nothing in Visual Basic).</exception>
		[SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public TransformManyBlock(Func<TInput, Task<IEnumerable<TOutput>>> transform) :
			this(null, transform, ExecutionDataflowBlockOptions.Default)
		{ }

		/// <summary>Initializes the <see cref="TransformManyBlock{TInput,TOutput}"/> with the specified function and <see cref="ExecutionDataflowBlockOptions"/>.</summary>
		/// <param name="transform">
		/// The function to invoke with each data element received. All of the data asynchronously returned in the <see cref="System.Collections.Generic.IEnumerable{TOutput}"/>
		/// will be made available as output from this <see cref="TransformManyBlock{TInput,TOutput}"/>.
		/// </param>
		/// <param name="dataflowBlockOptions">The options with which to configure this <see cref="TransformManyBlock{TInput,TOutput}"/>.</param>
		/// <exception cref="System.ArgumentNullException">The <paramref name="transform"/> is null (Nothing in Visual Basic).</exception>
		/// <exception cref="System.ArgumentNullException">The <paramref name="dataflowBlockOptions"/> is null (Nothing in Visual Basic).</exception>
		[SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public TransformManyBlock(Func<TInput, Task<IEnumerable<TOutput>>> transform, ExecutionDataflowBlockOptions dataflowBlockOptions) :
			this(null, transform, dataflowBlockOptions)
		{ }

		/// <summary>Initializes the <see cref="TransformManyBlock{TInput,TOutput}"/> with the specified function and <see cref="ExecutionDataflowBlockOptions"/>.</summary>
		/// <param name="transformSync">The synchronous function to invoke with each data element received.</param>
		/// <param name="transformAsync">The asynchronous function to invoke with each data element received.</param>
		/// <param name="dataflowBlockOptions">The options with which to configure this <see cref="TransformManyBlock{TInput,TOutput}"/>.</param>
		/// <exception cref="System.ArgumentNullException">The <paramref name="transformSync"/> and <paramref name="transformAsync"/> are both null (Nothing in Visual Basic).</exception>
		/// <exception cref="System.ArgumentNullException">The <paramref name="dataflowBlockOptions"/> is null (Nothing in Visual Basic).</exception>
		private TransformManyBlock(Func<TInput, IEnumerable<TOutput>> transformSync, Func<TInput, Task<IEnumerable<TOutput>>> transformAsync, ExecutionDataflowBlockOptions dataflowBlockOptions)
		{
			// Validate arguments.  It's ok for the filterFunction to be null, but not the other parameters.
			if (transformSync == null && transformAsync == null) throw new ArgumentNullException("transform");
			if (dataflowBlockOptions == null) throw new ArgumentNullException("dataflowBlockOptions");

			Contract.Requires(transformSync == null ^ transformAsync == null, "Exactly one of transformSync and transformAsync must be null.");
			Contract.EndContractBlock();

			// Ensure we have options that can't be changed by the caller
			dataflowBlockOptions = dataflowBlockOptions.DefaultOrClone();

			// Initialize onItemsRemoved delegate if necessary
			Action<ISourceBlock<TOutput>, int> onItemsRemoved = null;
			if (dataflowBlockOptions.BoundedCapacity > 0)
				onItemsRemoved = (owningSource, count) => ((TransformManyBlock<TInput, TOutput>)owningSource)._target.ChangeBoundingCount(-count);

			// Initialize source component
			_source = new SourceCore<TOutput>(this, dataflowBlockOptions,
					owningSource => ((TransformManyBlock<TInput, TOutput>)owningSource)._target.Complete(exception: null, dropPendingMessages: true),
					onItemsRemoved);

			// If parallelism is employed, we will need to support reordering messages that complete out-of-order.
			if (dataflowBlockOptions.SupportsParallelExecution)
			{
				_reorderingBuffer = new ReorderingBuffer<IEnumerable<TOutput>>(
						this, (source, messages) => ((TransformManyBlock<TInput, TOutput>)source)._source.AddMessages(messages));
			}

			// Create the underlying target and source
			if (transformSync != null) // sync
			{
				// If an enumerable function was provided, we can use synchronous completion, meaning
				// that the target will consider a message fully processed as soon as the
				// delegate returns.
				_target = new TargetCore<TInput>(this,
						messageWithId => ProcessMessage(transformSync, messageWithId),
						_reorderingBuffer, dataflowBlockOptions, TargetCoreOptions.None);
			}
			else // async
			{
				Debug.Assert(transformAsync != null, "Incorrect delegate type.");

				// If a task-based function was provided, we need to use asynchronous completion, meaning
				// that the target won't consider a message completed until the task
				// returned from that delegate has completed.
				_target = new TargetCore<TInput>(this,
						messageWithId => ProcessMessageWithTask(transformAsync, messageWithId),
						_reorderingBuffer, dataflowBlockOptions, TargetCoreOptions.UsesAsyncCompletion);
			}

			// Link up the target half with the source half.  In doing so, 
			// ensure exceptions are propagated, and let the source know no more messages will arrive.
			// As the target has completed, and as the target synchronously pushes work
			// through the reordering buffer when async processing completes, 
			// we know for certain that no more messages will need to be sent to the source.
#if NET_4_0_ABOVE
			_target.Completion.ContinueWith((completed, state) =>
			{
				var sourceCore = (SourceCore<TOutput>)state;
				if (completed.IsFaulted) sourceCore.AddAndUnwrapAggregateException(completed.Exception);
				sourceCore.Complete();
			}, _source, CancellationToken.None, Common.GetContinuationOptions(), TaskScheduler.Default);
#else
			Action<Task> continuationAction1 = completed =>
			{
				if (completed.IsFaulted) _source.AddAndUnwrapAggregateException(completed.Exception);
				_source.Complete();
			};
			_target.Completion.ContinueWith(continuationAction1, CancellationToken.None, Common.GetContinuationOptions(), TaskScheduler.Default);
#endif

			// It is possible that the source half may fault on its own, e.g. due to a task scheduler exception.
			// In those cases we need to fault the target half to drop its buffered messages and to release its 
			// reservations. This should not create an infinite loop, because all our implementations are designed
			// to handle multiple completion requests and to carry over only one.
#if NET_4_0_ABOVE
			_source.Completion.ContinueWith((completed, state) =>
			{
				var thisBlock = ((TransformManyBlock<TInput, TOutput>)state) as IDataflowBlock;
				Debug.Assert(completed.IsFaulted, "The source must be faulted in order to trigger a target completion.");
				thisBlock.Fault(completed.Exception);
			}, this, CancellationToken.None, Common.GetContinuationOptions() | TaskContinuationOptions.OnlyOnFaulted, TaskScheduler.Default);
#else
			Action<Task> continuationAction2 = completed =>
			{
				var thisBlock = this as IDataflowBlock;
				Debug.Assert(completed.IsFaulted, "The source must be faulted in order to trigger a target completion.");
				thisBlock.Fault(completed.Exception);
			};
			_source.Completion.ContinueWith(continuationAction2, CancellationToken.None, Common.GetContinuationOptions() | TaskContinuationOptions.OnlyOnFaulted, TaskScheduler.Default);
#endif

			// Handle async cancellation requests by declining on the target
			Common.WireCancellationToComplete(
					dataflowBlockOptions.CancellationToken, Completion, state => ((TargetCore<TInput>)state).Complete(exception: null, dropPendingMessages: true), _target);
#if FEATURE_TRACING
			DataflowEtwProvider etwLog = DataflowEtwProvider.Log;
			if (etwLog.IsEnabled())
			{
				etwLog.DataflowBlockCreated(this, dataflowBlockOptions);
			}
#endif
		}

		/// <summary>Processes the message with a user-provided transform function that returns an enumerable.</summary>
		/// <param name="transformFunction">The transform function to use to process the message.</param>
		/// <param name="messageWithId">The message to be processed.</param>
		private void ProcessMessage(Func<TInput, IEnumerable<TOutput>> transformFunction, KeyValuePair<TInput, long> messageWithId)
		{
			Contract.Requires(transformFunction != null, "Function to invoke is required.");

			bool userDelegateSucceeded = false;
			try
			{
				// Run the user transform and store the results.
				IEnumerable<TOutput> outputItems = transformFunction(messageWithId.Key);
				userDelegateSucceeded = true;
				StoreOutputItems(messageWithId, outputItems);
			}
			catch (Exception exc)
			{
				// If this exception represents cancellation, swallow it rather than shutting down the block.
				if (!Common.IsCooperativeCancellation(exc)) throw;
			}
			finally
			{
				// If the user delegate failed, store an empty set in order 
				// to update the bounding count and reordering buffer.
				if (!userDelegateSucceeded) StoreOutputItems(messageWithId, null);
			}
		}

		/// <summary>Processes the message with a user-provided transform function that returns an observable.</summary>
		/// <param name="function">The transform function to use to process the message.</param>
		/// <param name="messageWithId">The message to be processed.</param>
		[SuppressMessage("Microsoft.Design", "CA1031:DoNotCatchGeneralExceptionTypes")]
		private void ProcessMessageWithTask(Func<TInput, Task<IEnumerable<TOutput>>> function, KeyValuePair<TInput, long> messageWithId)
		{
			Contract.Requires(function != null, "Function to invoke is required.");

			// Run the transform function to get the resulting task
			Task<IEnumerable<TOutput>> task = null;
			Exception caughtException = null;
			try
			{
				task = function(messageWithId.Key);
			}
			catch (Exception exc) { caughtException = exc; }

			// If no task is available, either because null was returned or an exception was thrown, we're done.
			if (task == null)
			{
				// If we didn't get a task because an exception occurred, store it 
				// (or if the exception was cancellation, just ignore it).
				if (caughtException != null && !Common.IsCooperativeCancellation(caughtException))
				{
					Common.StoreDataflowMessageValueIntoExceptionData(caughtException, messageWithId.Key);
					_target.Complete(caughtException, dropPendingMessages: true, storeExceptionEvenIfAlreadyCompleting: true, unwrapInnerExceptions: false);
				}

				// Notify that we're done with this input and that we got no output for the input.
				if (_reorderingBuffer != null)
				{
					// If there's a reordering buffer, "store" an empty output.  This will
					// internally both update the output buffer and decrement the bounding count
					// accordingly.
					StoreOutputItems(messageWithId, null);
					_target.SignalOneAsyncMessageCompleted();
				}
				else
				{
					// As a fast path if we're not reordering, decrement the bounding
					// count as part of our signaling that we're done, since this will 
					// internally take the lock only once, whereas the above path will
					// take the lock twice.
					_target.SignalOneAsyncMessageCompleted(boundingCountChange: -1);
				}
				return;
			}

			// We got back a task.  Now wait for it to complete and store its results.
			// Unlike with TransformBlock and ActionBlock, We run the continuation on the user-provided 
			// scheduler as we'll be running user code through enumerating the returned enumerable.
#if NET_4_0_ABOVE
			task.ContinueWith((completed, state) =>
			{
				var tuple = (Tuple<TransformManyBlock<TInput, TOutput>, KeyValuePair<TInput, long>>)state;
				tuple.Item1.AsyncCompleteProcessMessageWithTask(completed, tuple.Item2);
			}, Tuple.Create(this, messageWithId),
			CancellationToken.None,
			Common.GetContinuationOptions(TaskContinuationOptions.ExecuteSynchronously),
			_source.DataflowBlockOptions.TaskScheduler);
#else
			Action<Task<IEnumerable<TOutput>>> continuationAction = completed =>
			{
				AsyncCompleteProcessMessageWithTask(completed, messageWithId);
			};
			task.ContinueWith(continuationAction, CancellationToken.None,
			Common.GetContinuationOptions(TaskContinuationOptions.ExecuteSynchronously),
			_source.DataflowBlockOptions.TaskScheduler);
#endif
		}

		/// <summary>Completes the processing of an asynchronous message.</summary>
		/// <param name="completed">The completed task storing the output data generated for an input message.</param>
		/// <param name="messageWithId">The originating message</param>
		[SuppressMessage("Microsoft.Design", "CA1031:DoNotCatchGeneralExceptionTypes")]
		private void AsyncCompleteProcessMessageWithTask(
				Task<IEnumerable<TOutput>> completed, KeyValuePair<TInput, long> messageWithId)
		{
			Contract.Requires(completed != null, "A task should have been provided.");
			Contract.Requires(completed.IsCompleted, "The task should have been in a final state.");

			switch (completed.Status)
			{
				case TaskStatus.RanToCompletion:
					IEnumerable<TOutput> outputItems = completed.Result;
					try
					{
						// Get the resulting enumerable and persist it.
						StoreOutputItems(messageWithId, outputItems);
					}
					catch (Exception exc)
					{
						// Enumerating the user's collection failed. If this exception represents cancellation, 
						// swallow it rather than shutting down the block.
						if (!Common.IsCooperativeCancellation(exc))
						{
							// The exception was not for cancellation. We must add the exception before declining 
							// and signaling completion, as the exception is part of the operation, and the completion 
							// conditions depend on this.
							Common.StoreDataflowMessageValueIntoExceptionData(exc, messageWithId.Key);
							_target.Complete(exc, dropPendingMessages: true, storeExceptionEvenIfAlreadyCompleting: true, unwrapInnerExceptions: false);
						}
					}
					break;

				case TaskStatus.Faulted:
					// We must add the exception before declining and signaling completion, as the exception 
					// is part of the operation, and the completion conditions depend on this.
					AggregateException aggregate = completed.Exception;
					Common.StoreDataflowMessageValueIntoExceptionData(aggregate, messageWithId.Key, targetInnerExceptions: true);
					_target.Complete(aggregate, dropPendingMessages: true, storeExceptionEvenIfAlreadyCompleting: true, unwrapInnerExceptions: true);
					goto case TaskStatus.Canceled;
				case TaskStatus.Canceled:
					StoreOutputItems(messageWithId, null); // notify the reordering buffer and decrement the bounding count
					break;

				default:
					Debug.Assert(false, "The task should have been in a final state.");
					break;
			}

			// Let the target know that one of the asynchronous operations it launched has completed.
			_target.SignalOneAsyncMessageCompleted();
		}

		/// <summary>
		/// Stores the output items, either into the reordering buffer or into the source half.
		/// Ensures that the bounding count is correctly updated.
		/// </summary>
		/// <param name="messageWithId">The message with id.</param>
		/// <param name="outputItems">The output items to be persisted.</param>
		private void StoreOutputItems(
				KeyValuePair<TInput, long> messageWithId, IEnumerable<TOutput> outputItems)
		{
			// If there's a reordering buffer, pass the data along to it.
			// The reordering buffer will handle all details, including bounding.
			if (_reorderingBuffer != null)
			{
				StoreOutputItemsReordered(messageWithId.Value, outputItems);
			}
			// Otherwise, output the data directly.
			else if (outputItems != null)
			{
				// If this is a trusted type, output the data en mass.
				if (outputItems is TOutput[] || outputItems is List<TOutput>)
				{
					StoreOutputItemsNonReorderedAtomic(outputItems);
				}
				else
				{
					// Otherwise, we need to take the slow path of enumerating
					// each individual item.
					StoreOutputItemsNonReorderedWithIteration(outputItems);
				}
			}
			else if (_target.IsBounded)
			{
				// outputItems is null and there's no reordering buffer
				// and we're bounding, so decrement the bounding count to
				// signify that the input element we already accounted for
				// produced no output
				_target.ChangeBoundingCount(count: -1);
			}
			// else there's no reordering buffer, there are no output items, and we're not bounded,
			// so there's nothing more to be done.
		}

		/// <summary>Stores the next item using the reordering buffer.</summary>
		/// <param name="id">The ID of the item.</param>
		/// <param name="item">The completed item.</param>
		private void StoreOutputItemsReordered(long id, IEnumerable<TOutput> item)
		{
			Contract.Requires(_reorderingBuffer != null, "Expected a reordering buffer");
			Contract.Requires(id != Common.INVALID_REORDERING_ID, "This ID should never have been handed out.");

			// Grab info about the transform
			TargetCore<TInput> target = _target;
			bool isBounded = target.IsBounded;

			// Handle invalid items (null enumerables) by delegating to the base
			if (item == null)
			{
				_reorderingBuffer.AddItem(id, null, false);
				if (isBounded) target.ChangeBoundingCount(count: -1);
				return;
			}

			// If we can eagerly get the number of items in the collection, update the bounding count.
			// This avoids the cost of updating it once per output item (since each update requires synchronization).
			// Even if we're not bounding, we still want to determine whether the item is trusted so that we 
			// can immediately dump it out once we take the lock if we're the next item.
			IList<TOutput> itemAsTrustedList = item as TOutput[];
			if (itemAsTrustedList == null) itemAsTrustedList = item as List<TOutput>;
			if (itemAsTrustedList != null && isBounded)
			{
				UpdateBoundingCountWithOutputCount(count: itemAsTrustedList.Count);
			}

			// Determine whether this id is the next item, and if it is and if we have a trusted list,
			// try to output it immediately on the fast path.  If it can be output, we're done.
			// Otherwise, make forward progress based on whether we're next in line.
			bool? isNextNullable = _reorderingBuffer.AddItemIfNextAndTrusted(id, itemAsTrustedList, itemAsTrustedList != null);
			if (!isNextNullable.HasValue) return; // data was successfully output
			bool isNextItem = isNextNullable.Value;

			// By this point, either we're not the next item, in which case we need to make a copy of the
			// data and store it, or we are the next item and can store it immediately but we need to enumerate
			// the items and store them individually because we don't want to enumerate while holding a lock.
			List<TOutput> itemCopy = null;
			try
			{
				// If this is the next item, we can output it now.
				if (isNextItem)
				{
					StoreOutputItemsNonReorderedWithIteration(item);
					// here itemCopy remains null, so that base.AddItem will finish our interactions with the reordering buffer
				}
				else if (itemAsTrustedList != null)
				{
					itemCopy = itemAsTrustedList.ToList();
					// we already got the count and updated the bounding count previously
				}
				else
				{
					// We're not the next item, and we're not trusted, so copy the data into a list.
					// We need to enumerate outside of the lock in the base class.
					int itemCount = 0;
					try
					{
						itemCopy = item.ToList(); // itemCopy will remain null in the case of exception
						itemCount = itemCopy.Count;
					}
					finally
					{
						// If we're here successfully, then itemCount is the number of output items
						// we actually received, and we should update the bounding count with it.
						// If we're here because ToList threw an exception, then itemCount will be 0,
						// and we still need to update the bounding count with this in order to counteract
						// the increased bounding count for the corresponding input.
						if (isBounded) UpdateBoundingCountWithOutputCount(count: itemCount);
					}
				}
				// else if the item isn't valid, the finally block will see itemCopy as null and output invalid
			}
			finally
			{
				// Tell the base reordering buffer that we're done.  If we already output
				// all of the data, itemCopy will be null, and we just pass down the invalid item.  
				// If we haven't, pass down the real thing.  We do this even in the case of an exception,
				// in which case this will be a dummy element.
				_reorderingBuffer.AddItem(id, itemCopy, itemIsValid: itemCopy != null);
			}
		}

		/// <summary>
		/// Stores the trusted enumerable en mass into the source core.
		/// This method does not go through the reordering buffer.
		/// </summary>
		/// <param name="outputItems"></param>
		private void StoreOutputItemsNonReorderedAtomic(IEnumerable<TOutput> outputItems)
		{
			Contract.Requires(_reorderingBuffer == null, "Expected not to have a reordering buffer");
			Contract.Requires(outputItems is TOutput[] || outputItems is List<TOutput>, "outputItems must be a list we've already vetted as trusted");
			if (_target.IsBounded) UpdateBoundingCountWithOutputCount(count: ((ICollection<TOutput>)outputItems).Count);
			_source.AddMessages(outputItems);
		}

		/// <summary>
		/// Stores the untrusted enumerable into the source core.
		/// This method does not go through the reordering buffer.
		/// </summary>
		/// <param name="outputItems">The untrusted enumerable.</param>
		private void StoreOutputItemsNonReorderedWithIteration(IEnumerable<TOutput> outputItems)
		{
			// If we're bounding, we need to increment the bounded count
			// for each individual item as we enumerate it.
			if (_target.IsBounded)
			{
				// When the input item that generated this
				// output was loaded, we incremented the bounding count.  If it only
				// output a single a item, then we don't need to touch the bounding count.
				// Otherwise, we need to adjust the bounding count accordingly.
				bool outputFirstItem = false;
				try
				{
					foreach (TOutput item in outputItems)
					{
						if (outputFirstItem) _target.ChangeBoundingCount(count: 1);
						else outputFirstItem = true;
						_source.AddMessage(item);
					}
				}
				finally
				{
					if (!outputFirstItem) _target.ChangeBoundingCount(count: -1);
				}
			}
			// If we're not bounding, just output each individual item.
			else
			{
				foreach (TOutput item in outputItems) _source.AddMessage(item);
			}
		}

		/// <summary>
		/// Updates the bounding count based on the number of output items
		/// generated for a single input.
		/// </summary>
		/// <param name="count">The number of output items.</param>
		private void UpdateBoundingCountWithOutputCount(int count)
		{
			// We already incremented the count for a single input item, and
			// that input spawned 0 or more outputs.  Take the input tracking
			// into account when figuring out how much to increment or decrement
			// the bounding count.

			Contract.Requires(_target.IsBounded, "Expected to be in bounding mode.");
			if (count > 1) _target.ChangeBoundingCount(count - 1);
			else if (count == 0) _target.ChangeBoundingCount(-1);
			else Debug.Assert(count == 1, "Count shouldn't be negative.");
		}

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Blocks/Member[@name="Complete"]/*' />
		public void Complete() { _target.Complete(exception: null, dropPendingMessages: false); }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Blocks/Member[@name="Fault"]/*' />
		void IDataflowBlock.Fault(Exception exception)
		{
			if (exception == null) throw new ArgumentNullException("exception");
			Contract.EndContractBlock();

			_target.Complete(exception, dropPendingMessages: true);
		}

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="LinkTo"]/*' />
		public IDisposable LinkTo(ITargetBlock<TOutput> target, DataflowLinkOptions linkOptions) { return _source.LinkTo(target, linkOptions); }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="TryReceive"]/*' />
		public Boolean TryReceive(Predicate<TOutput> filter, out TOutput item) { return _source.TryReceive(filter, out item); }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="TryReceiveAll"]/*' />
		public bool TryReceiveAll(out IList<TOutput> items) { return _source.TryReceiveAll(out items); }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Blocks/Member[@name="Completion"]/*' />
		public Task Completion { get { return _source.Completion; } }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Targets/Member[@name="InputCount"]/*' />
		public int InputCount { get { return _target.InputCount; } }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="OutputCount"]/*' />
		public int OutputCount { get { return _source.OutputCount; } }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Targets/Member[@name="OfferMessage"]/*' />
		DataflowMessageStatus ITargetBlock<TInput>.OfferMessage(DataflowMessageHeader messageHeader, TInput messageValue, ISourceBlock<TInput> source, Boolean consumeToAccept)
		{
			return _target.OfferMessage(messageHeader, messageValue, source, consumeToAccept);
		}

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="ConsumeMessage"]/*' />
		TOutput ISourceBlock<TOutput>.ConsumeMessage(DataflowMessageHeader messageHeader, ITargetBlock<TOutput> target, out Boolean messageConsumed)
		{
			return _source.ConsumeMessage(messageHeader, target, out messageConsumed);
		}

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="ReserveMessage"]/*' />
		bool ISourceBlock<TOutput>.ReserveMessage(DataflowMessageHeader messageHeader, ITargetBlock<TOutput> target)
		{
			return _source.ReserveMessage(messageHeader, target);
		}

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Sources/Member[@name="ReleaseReservation"]/*' />
		void ISourceBlock<TOutput>.ReleaseReservation(DataflowMessageHeader messageHeader, ITargetBlock<TOutput> target)
		{
			_source.ReleaseReservation(messageHeader, target);
		}

		/// <summary>Gets the number of messages waiting to be processed.  This must only be used from the debugger as it avoids taking necessary locks.</summary>
		private int InputCountForDebugger { get { return _target.GetDebuggingInformation().InputCount; } }
		/// <summary>Gets the number of messages waiting to be processed.  This must only be used from the debugger as it avoids taking necessary locks.</summary>
		private int OutputCountForDebugger { get { return _source.GetDebuggingInformation().OutputCount; } }

		/// <include file='..\XmlDocs\CommonXmlDocComments.xml' path='CommonXmlDocComments/Blocks/Member[@name="ToString"]/*' />
		public override string ToString() { return Common.GetNameForDebugger(this, _source.DataflowBlockOptions); }

		/// <summary>The data to display in the debugger display attribute.</summary>
		[SuppressMessage("Microsoft.Globalization", "CA1305:SpecifyIFormatProvider")]
		private object DebuggerDisplayContent
		{
			get
			{
				return string.Format("{0}, InputCount={1}, OutputCount={2}",
						Common.GetNameForDebugger(this, _source.DataflowBlockOptions),
						InputCountForDebugger,
						OutputCountForDebugger);
			}
		}
		/// <summary>Gets the data to display in the debugger display attribute for this instance.</summary>
		object IDebuggerDisplay.Content { get { return DebuggerDisplayContent; } }

		/// <summary>Provides a debugger type proxy for the TransformManyBlock.</summary>
		private sealed class DebugView
		{
			/// <summary>The transform many block being viewed.</summary>
			private readonly TransformManyBlock<TInput, TOutput> _transformManyBlock;
			/// <summary>The target half of the block being viewed.</summary>
			private readonly TargetCore<TInput>.DebuggingInformation _targetDebuggingInformation;
			/// <summary>The source half of the block being viewed.</summary>
			private readonly SourceCore<TOutput>.DebuggingInformation _sourceDebuggingInformation;

			/// <summary>Initializes the debug view.</summary>
			/// <param name="transformManyBlock">The transform being viewed.</param>
			public DebugView(TransformManyBlock<TInput, TOutput> transformManyBlock)
			{
				Contract.Requires(transformManyBlock != null, "Need a block with which to construct the debug view.");
				_transformManyBlock = transformManyBlock;
				_targetDebuggingInformation = transformManyBlock._target.GetDebuggingInformation();
				_sourceDebuggingInformation = transformManyBlock._source.GetDebuggingInformation();
			}

			/// <summary>Gets the messages waiting to be processed.</summary>
			public IEnumerable<TInput> InputQueue { get { return _targetDebuggingInformation.InputQueue; } }
			/// <summary>Gets any postponed messages.</summary>
			public QueuedMap<ISourceBlock<TInput>, DataflowMessageHeader> PostponedMessages { get { return _targetDebuggingInformation.PostponedMessages; } }
			/// <summary>Gets the messages waiting to be received.</summary>
			public IEnumerable<TOutput> OutputQueue { get { return _sourceDebuggingInformation.OutputQueue; } }

			/// <summary>Gets the number of input operations currently in flight.</summary>
			public Int32 CurrentDegreeOfParallelism { get { return _targetDebuggingInformation.CurrentDegreeOfParallelism; } }
			/// <summary>Gets the task being used for output processing.</summary>
			public Task TaskForOutputProcessing { get { return _sourceDebuggingInformation.TaskForOutputProcessing; } }

			/// <summary>Gets the DataflowBlockOptions used to configure this block.</summary>
			public ExecutionDataflowBlockOptions DataflowBlockOptions { get { return _targetDebuggingInformation.DataflowBlockOptions; } }
			/// <summary>Gets whether the block is declining further messages.</summary>
			public bool IsDecliningPermanently { get { return _targetDebuggingInformation.IsDecliningPermanently; } }
			/// <summary>Gets whether the block is completed.</summary>
			public bool IsCompleted { get { return _sourceDebuggingInformation.IsCompleted; } }
			/// <summary>Gets the block's Id.</summary>
			public int Id { get { return Common.GetBlockId(_transformManyBlock); } }

			/// <summary>Gets the set of all targets linked from this block.</summary>
			public TargetRegistry<TOutput> LinkedTargets { get { return _sourceDebuggingInformation.LinkedTargets; } }
			/// <summary>Gets the set of all targets linked from this block.</summary>
			public ITargetBlock<TOutput> NextMessageReservedFor { get { return _sourceDebuggingInformation.NextMessageReservedFor; } }
		}
	}
}
